It's not seven years because Smith is going to Boston College next year. So, every year Smith ends up being in college plus every year he's in the NHL but isn't that good because he's still developing comes off of that 7 years. Right now it's 6 years max. And if he does like Cooley and stays in college two years, it goes down to 5. Then if he has early NHL seasons like say Slaf or KK, it goes down 4 or 3.
The point is we don't know when he'll start to be a good NHLer, just as we don't know how good of an NHLer he'll be. He may be a point per game NHLer right out of Boston College. Or, he may never even be an NHLer. Or, he may be anything in between such as the next Galchenyuk or Drouin, or a late bloomer like Jonathan Marchessault.
Whereas with Keller, you know he's just entered his prime and put up 37g and 86 points at 24 on a bad team. What could he do playing with Dach, or Dach and Dubois, When there's also another line on the team that has Caufield and Suzuki on it?